FIG. 1 shows a part of a telecommunication network 10. The radio access network 10 comprises a plurality of radio base stations 11 (whereof only one is shown in the figure), each of which communicates with a plurality of user equipments 12 located in the coverage area of the radio base station. The base station 11 further communicates with a core network 13. For example, where the radio access network 10 is a standardized E-UTRAN (Evolved UMTS Terrestrial Radio Access Network), the core network 13 comprises an evolved packet core, itself comprising a mobility management entity (MME), a serving gateway and a PDN (packet data network) gateway.
The E-UTRAN uses OFDM (Orthogonal Frequency Division Multiple Access) in the downlink and DFT (Discrete Fourier Transform) spread OFDM in the uplink. The LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2, where each resource element 20 including a cyclic prefix 21 corresponds to one OFDM subcarrier during one OFDM symbol interval.
The E-UTRAN time-domain structure is illustrated in FIG. 3. In the time domain, downlink transmissions are organized into radio frames of 10 ms, each radio frame 30 consisting of ten equally-sized subframes 31 with a length of 1 ms. Each subframe consists of two slots of 0.5 ms and each slot consists of a number of OFDM symbols.
Furthermore, resource allocation in E-UTRAN is typically described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink (DL) transmissions could be dynamically scheduled. That is, in each subframe the radio base station transmits control information, indicating to which user equipments data is transmitted and upon which resource blocks the data is transmitted, i.e. the DL assignment, in the current downlink subframe. The DL assignment is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. In FIG. 4 a downlink subframe 40 comprising a control region 41 is illustrated. The control region 41 includes 3 OFDM symbols 42 comprising control information. The DL assignment is transmitted in a resource allocation message as a Downlink Control Information (DCI) message on the Physical Downlink Control CHannel (PDCCH) within the control region. The UL grant is also transmitted in a resource allocation message as a Downlink Control Information (DCI) message on the Physical Downlink Control CHannel (PDCCH). Among other important parameters the DCI message contains information indicating the assigned resources for the Physical Downlink Shared CHannel (PDSCH) and UL transmission grants for the Physical Uplink Shared CHannel (PUSCH) in a resource allocation field. The size of the resource allocation field increases with the transmission bandwidth of the system, the wider the transmission bandwidth the more bits are needed to describe the resource allocation. Even though multiple resource allocation schemes exist in LTE this basic principle applies to all of them. In addition to the transmission bandwidth also the transmission mode (e.g. which kind of MIMO or no MIMO) influences the DCI payload size i.e. the size of the resource allocation field.
Each PDCCH is protected by a Cyclic Redundancy Check (CRC) and is Forward Error Correction (FEC) encoded. The CRC is masked by a pattern that depends on the UE identity of the recipient. If a PDCCH should be received by multiple terminals the scrambling does not depend on the UE identity but on a group identity, e.g. the Random Access group identity, Paging group identity, etc.
Depending on the Signal to Interference plus Noise Ratio (SINR) situation of the UE a higher or lower error protection is required for an acceptable decoding performance. Therefore the code rate of the error correction code can be adjusted, i.e. the same DCI payload size can be encoded into code words with different sizes.
For each subframe the UEs monitors the control region and tries to decode PDCCH. In order to reduce complexity the UE is only supposed to monitor PDCCH transmitted on a fraction of all available resource elements, so called search spaces. Furthermore, the UE only monitors DCI messages with a limited number of payload sizes (i.e. for a limited number of transmission modes), which translates via the different codes rates into a limited set of different code word sizes. Once it has decoded the PDCCH it calculates the CRC of the decoded payload, masks it with its own UE identity or an appropriate group identity and compares the result with the attached CRC. If the result matches the UE has decoded an assignment attended for it, otherwise it has decoded an assignment intended for someone else and discards it.
The E-UTRAN currently supports bandwidths up to 20 MHz. However, one of the requirements of future releases of this standard such as LTE Advanced is the support of bandwidths larger than 20 MHz. A further important future requirement is to assure backward compatibility with previous releases. This also includes spectrum compatibility. That would imply that a future-release carrier, wider than 20 MHz, appears as a number of carriers to a legacy UE. Each such carrier can be referred to as a component carrier. In particular for early deployments of future releases, it can be expected that there will be a smaller number of future-release UEs compared to many legacy UEs. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy UEs, i.e. that it is possible to implement carriers where legacy UEs can be scheduled in all parts of the wideband future-release carrier. The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that a future-release UE can receive multiple component carriers, where the component carriers have, or at least have the possibility of having, the same structure as a carrier of previous releases.
Carrier aggregation is illustrated in FIG. 5 where five component carriers 50, each of 20 MHz bandwidth, have been aggregated to form a bandwidth of 100 MHz. The number of aggregated component carriers as well as the bandwidth of the individual component carrier may be different for uplink (UL) and downlink (DL). A symmetric configuration refers to the case where the number of component carriers in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of component carriers is different. It should be noted that the number of component carriers configured in a coverage area may be different from the number of component carriers seen by a UE. A UE may for example support more DL component carriers than UL component carriers, even though the coverage area is configured with the same number of UL and DL component carriers.
A straight forward extension of current E-UTRAN is to use PDCCH transmitted on a certain component carrier to schedule the PDSCH, which is transmitted on the same component carrier. However, in certain scenarios it is desirable to enable cross-scheduling, i.e. PDCCH is transmitted on a first component carrier whereas the corresponding PDSCH resides on another component carrier. To identify the component carrier the PDSCH is located on a Carrier Indicator Field (CIF) is included in the PDCCH.
With above outlined CIF a particular component carrier can be used to schedule the PDSCH on multiple component carriers. If we assume the same transmission bandwidth on each component carrier as well as the same transmission mode, the DCI format sizes containing the assignments are the same for all component carriers. Thus, a UE has to monitor PDCCHs of certain sizes. After decoding and verifying that the assignment is indeed for the UE, the component carrier containing the actual assignment (i.e. PDSCH) is known from the CIF.
However, in case the component carriers that can be scheduled from one component carrier have different bandwidths also the payload sizes of the DCI messages vary across the component carriers. Moreover, for each payload size a certain number of code word sizes needs to be monitored by the UE, increasing the number of blind decodings. Thus, the component carriers having different bandwidths results in an increased number of blind decodings.
A state-of-the-art solution to this problem is to provide one resource allocation message to allocate resources on more than one component carrier. This is performed by allowing existing control signaling to indicate a larger set of resource blocks than in networks in which carrier aggregation is not applied. This solution results in reduced sizes of the resource allocation fields in the DCI messages addressing wider component carriers. The sizes are reduced to the size used on the component carrier carrying the PDCCH, resulting in coarser resource allocations on the cross-scheduled component carriers having wider bandwidths. For cross-scheduled component carriers having narrower bandwidths the resource allocation field is extended, resulting in finer resource allocations. However, there is a need for a solution when the scheduling component carrier is wide and the cross-scheduled component carriers are narrower, providing a smaller amount of overhead.